All you need to know is that you have to post your atom like this:
<?xml version="1.0" encoding="UTF-8"?>
<title mode="escaped" type="text/html">$title</title>
<content mode="escaped" type="text/html">$entry</content>
Also you have to know how to send a HTTP Digest Auth.
The formula is:
Where A1 = username:realm:password
A2 = Method:uri
H(data) = MD5(data)
KD(secret, data) = H(secret:data)
That is straight from RFC 2617
, and it works fine in LJ.
I actually implemented an atom posting client package in Perl, and it works with both LJ and Blogger. You can read about it on my blog
I've been using it for a few days now, and it works great. If you have any questions, just ask.